The present invention relates to a photoelectric conversion device that converts optical information such as light intensity, light wavelength, and the like, and an optical image into electronic information including current or voltage, or digital data. In particular, the present invention relates to a photoelectric conversion device having improved performance. Additionally, the present invention relates to a photoelectric conversion cell to which the present invention is applied, a photoelectric conversion array including an array architecture of the photoelectric cell, and an imaging device including the photoelectric conversion array.
In the technology utilized by the present invention where a base of a phototransistor is in an electrically floating state, and a photocurrent flowing through the base of the phototransistor is accumulated for a certain period of time, and is read out as an electric signal, in a case where a potential or charge accumulated in the base is not completely drawn out at the time of reading out, it is necessary to reset a residual potential or charge in the base after reading out (return to an initial state). In a case where the phototransistor is used as a component of an imaging device, the residual potential or charge adversely affects an imaging process.
Japanese Patent Application Publication number H01-198183 discloses an electrode provided in a base of a phototransistor, and a method for resetting a base potential to a constant value through the base electrode after reading out. Additionally, Japanese Patent Application Publication number H01-198183 discloses that an output linearity with respect to an exposure amount is improved by making a reset potential of an emitter or a base closer to a collector potential than a reset potential of a signal line.
However, when choosing a reset potential closer to a collector potential so as to obtain output even when an exposure amount is zero, there is a problem such that it is not possible to obtain sensitivity with respect to a small amount of light because there is a baseline output. On the other hand, when a reset potential is far from a collector potential, and output is not obtained when an exposure amount is zero or small, it is not possible to obtain output with respect to the small amount of light. Japanese Patent Application Publication number H01-198183 also indicates the latter problem; however, specific knowledge regarding an optimal reset potential is not disclosed. In addition, although setting of not an absolute value of a base potential, but a base-emitter voltage is essential, a method for resetting a base-emitter voltage, and specific knowledge regarding an optimal base-emitter voltage are not disclosed either.
Furthermore, due to the above reset mechanism, there is a problem such that a necessary switch circuit and interconnections necessarily increase an area of a photoelectric conversion device. Additionally, there is also a problem such that providing a base electrode disclosed in Japanese Patent Application Publication number H01-198183 causes an increase of a dark current, reduces efficiency by forming a shadow on a light-illuminated surface of a phototransistor, and increases the area of the photoelectric conversion device.
On the other hand, in a case where a base of a phototransistor is in an electrically floating state, and a capacitance (a base-collector junction capacitance, a base floating capacitance, etc.) connected to the base is charged and discharged by a photocurrent flowing through a base-collector junction of the phototransistor for a certain period of time, and a change of an accumulated electric charge quantity is read out as an electric signal, when the light intensity is large with respect to an accumulation time (equivalent to the certain period of time, and also referred to as an integration time), a change of voltage of the capacitance connected to the base becomes too large, the photocurrent flows through the base-collector junction in the forward direction, the base-collector junction is deeply biased to a deep forward voltage, and in each region of two opposite-conductivity-type semiconductors constituting the base-collector junction, excess minority carriers are stored. Therefore, when switching the base-collector junction in a reverse-biased direction, there is a problem such that the time taken to switch in the reverse-biased direction is delayed during a time called “a saturation time” related to a minority carrier lifetime, and a response speed is affected. This phenomenon is called a “saturation phenomenon”.
As a saturation control technique, a technique to prevent a base-collector junction from being deeply biased to a deep forward voltage, namely, a technique where a Schottky junction is connected in parallel to the base-collector junction is known (see Japanese Patent Publication number S47-18561). Compared to cases where the same current flows through the base-collector junction and the Schottky junction, a forward voltage is smaller in the case of the Schottky junction, and therefore most current flows not through the base-collector junction but through the Schhottky junction which is connected in parallel, and thus it is possible to prevent the base-collector junction from being deeply forward biased. However, a reverse current of the Schottky junction is orders of magnitude larger than that of the base-collector junction; therefore, an overall dark current as a photoelectric conversion element increases, and it is unusable for a high-sensitivity photoelectric conversion device.
The objects of the present invention are: 1. provision of a method of setting a base-emitter reset voltage and an optimal voltage for the base-emitter reset; 2. removal of a base electrode; and 3. provision of a saturation control method utilizing a switch circuit and interconnections that are used for a reset function with increase in an area of a photoelectric conversion device.
The saturation control technique prevents a saturation state where a base-collector junction is “deeply biased to a deep forward voltage”; however, that is a technique where a reverse current of the base-collector junction does not become as large as that of the Schottky junction.
Note that in the present invention, “deeply biased to a forward voltage” or “deeply biased to a deep forward voltage” means that a state of a forward voltage of a base-collector junction when a photocurrent flows to the forward direction all through the base-collector junction, for example, in a state where only equal to or less than 1/10 of the photocurrent flows, a saturation time is improved to about 1/10; therefore, it is considered that a problem has been solved. A forward voltage of the base-collector junction in that case is smaller than a state of “deeply biased to the deep forward voltage” by only 2.3 kT/q (=about 60 mV at a normal temperature). In the present invention, “a saturation control” is that in a circumstance where a PN junction is deeply biased to a deep forward voltage when being left, the base-collector junction is controlled in a forward-biased state smaller by only 2.3 kT/q than the deep forward voltage state, or the base-collector junction is kept in a zero-biased state, or in a reverse-biased state. Here, k denotes Boltzmann constant, T denotes an absolute temperature of the photoelectric conversion device, and q denotes elementary charge of an electron.
In a case where the base-collector junction is formed by providing a second semiconductor region of an opposite conductivity type in contact with a first semiconductor region of a first conductivity type, those stored excess minority carriers spread from a position of the base-collector junction within a range of a minority carrier diffusion length in the first semiconductor region. Likewise, also in the second semiconductor region, those stored excess minority carriers spread from the base-collector junction within the range of minority carrier diffusion length. Note that the minority carrier diffusion length is different from carrier types (electron or hole), and an electric characteristic of a semiconductor region. Therefore, values of the diffusion length are different between the first semiconductor region and the second semiconductor region.
Furthermore, when another base-collector junction having a photoelectric conversion function is provided at a distance of the diffusion length, electric current flows through the other base-collector junction even though light does not illuminate the other base-collector junction, and malfunction occurs as if light illuminated a position of the other base-collector junction. In a case where in the first semiconductor region, many photoelectric conversion elements such as a base-collector junction having a photoelectric conversion function are arranged, and a photoelectric conversion element array is formed, and an imaging device is made, an image blur occurs and resolution declines equivalently, and as a result, the above problem occurs.
One of objects of the present invention is to improve an image blur and equivalent deterioration of the resolution by effectively utilizing a switch circuit and interconnections that are provided for the reset function but increase an area of the photoelectric conversion device.
Note that storage of excess minority carriers (electric charges) causing saturation, and charging and discharging of a base capacitance by a photocurrent as electronic information (for example, base-collector junction capacitance Cbc, base-emitter junction capacitance Cbe, base-ground capacitance Cbst) are different phenomena. In the present invention, a change of a charge amount of the base capacitance occurring as a result of charging or discharging of the base capacitance by the photocurrent is called “accumulation charge by light”.